The present invention relates to an apparatus for reproducing recorded information from a recording disc, and more particularly to a time base control unit in an apparatus for reproducing recorded information from an information recording disc such as a video disc, digital audio disc or the like in which a video signal, a PCM digital audio signal or the like, which disc has playback mode functions which require jumping operations such as still (STILL), search (SEARCH), slow (SLOW), or fast (FAST), etc.
In general, there is known a disc reproducing apparatus for reproducing an information recording disc (referred to disc simply hereinbelow) such as a color video disc, a digital audio disc or the like in which a color video signal including video information such as a composite color video signal and so on, or a PCM digital audio signal which had been time base compressed has been recorded optically, electrostatically, electromagnetically, or mechanically. Such a disc reproducing apparatus as mentioned above generally has an ordinary sequential playback mode function and other playback mode functions such as still (still picture) playback, fast (interger multiple speed) playback or the like which require the jumping operation for jumping a read-spot (information detection area) from one track to another track which is one pitch or more apart therefrom.
FIGS. 1(A), (B) and (C) illustrate the three kinds of transit traces of a read-spot on track T of a disc 1 in case of a sequential playback, still picture playback and three times normal playback.
On the other hand, in a disc reproducing apparatus, the time base fluctuation is noted in reproduced signals derived from the read-pickup due to wow and flutter in a disc rotation driving motor (i.e. spindle motor) rotating the disc at a constant speed, and off-center disc, or the like. To restrain such time base fluctuation in the reproduction signal, a spindle servo or tangential servo is applied. However, in order to perform such spindle servo or tangential servo control, the following method has been done conventionally; that is, a reproduction synchronization signal (refered sync signal hereinbelow) such as horizontal synchronization pulse (refered sync pulse hereinbelow) or the like is extracted from a video signal to be obtained from the disc, and a phase difference signal is generated which is indicative of a phase difference between this reproduction sync signal and a reference sync signal to be generated in the recording disc reproducing apparatus, then the rotating speed of the spindle motor is controlled or the tangential velocity of a recording track on the disc of the read-spot is controlled so that the phase difference represented by the above-mentioned phase difference signal becomes small.
An embodiment of a spindle servo circuit as a time base control circuit by the above-stated phase difference signal will be described with reference to FIG. 2, in which the video disc 1 is rotated by a spindle motor 2. In case of a CAV type video disc, the spindle motor 2 rotates the video disc 1 at a constant angular velocity, and in case of a CLV type video disc, it rotates the video disc 1 so that the tangential velocity on the track of a read-spOt of a read-pickup 3 is always constant. The read-pickup 3 is positioned by a tracking servo system 4 so as to trace a desired track. In a particular playback mode which requires the jumping operation such as still playback or the like, the tracking servo system 4 serves to jump the read-spot of the read-pickup 3 according to a jump command to be supplied from a control circuit (not shown). An example of such a control circuit for generating a jump command is disclosed in U.S. Pat. No. 4,403,259 filed Sept. 28, 1981.
The signal derived from the above read-pickup 3 is demodulated by a demodulator 5 to produce a video signal. This video signal is fed to a picture reproduction circuit (not shown), which circuit is well known, so its detail is omitted herein. On the other hand, a separator 6 separates a reproduction sync signal such as a horizontal sync pulse or the like which is included in the video signal, and supplies to an input terminal of a phase comparator 7. The phase comparator 7 receives at the other input terminal a reference sync signal of a prescribed frequency from a reference sync signal generator 8. The phase comparator 7 generates a phase difference signal indicative of a difference of the phases between the reference sync signal and the reproduction sync signal, and supplies to a spindle servo circuit 10 through a sample holder 9. The spindle servo circuit 10 controls the rotating speed of the spindle motor in response to this phase difference signal to execute the spindle servo operation.
As described above, in a conventional disc reproducing apparatus, the time base fluctuation of the information signal read from the disc is detected by way of a phase difference between the sync signal included in the reproduction information signal and the reference sync signal of a prescribed frequency. In the case where a CLV-type disc is played back and the jumping operation is performed by such a disc reproducing apparatus as mentioned above, the sections where sync signals have been recorded in the CLV-type disc are not arranged in the radial direction, therefore, fairly a long time is required until the time base servo system becomes stable. Furthermore, in an extreme case, the time base servo system becomes out of order due to too much big disturbance caused by the jumping operation.